1. Field of the Invention
This invention relates generally to superconducting quantum interference devices (SQUIDs) and more specifically to high-transition-temperature, low-noise SQUIDs.
2. Description of Related Art
Superconducting quantum interference devices (SQUIDs) are used to detect magnetic flux. They transduce magnetic flux into voltage. The SQUID is used for a range of applications in which low level physical phenomena are readily converted to magnetic flux. For example, current, voltage, magnetic field, and magnetic field gradient can all be measured via magnetic flux. High transition temperature (high-T.sub.C) SQUIDs are those made with materials having a transition temperature above the boiling point of liquid nitrogen (77 K). Low-transition temperature (low-T.sub.C) SQUIDs are those made with materials having a transition temperature below the boiling point of liquid nitrogen (77 K). A SQUID's usefulness is limited primarily by its intrinsic noise, the ability to efficiently couple a signal to it, and by the ability to read out a signal.
Magnetometers based on low-T.sub.C SQUIDs exhibit extremely high sensitivity, broad bandwidth (up to tens of GigaHz), and small physical size. However the low temperature requirement, necessitating liquid helium, makes low-T.sub.C SQUIDs expensive and cumbersome to use. It would thus be very desirable to fabricate high-T.sub.C SQUIDs that had some of the desirable performance characteristics of the low-T.sub.C counterparts. Unfortunately, however, in high-T.sub.C SQUIDs the level of 1/f noise tends to be impractically high, particularly when cooled and operated in the presence of a magnetic field, even one as small as the Earth's ambient magnetic field, which is about 50 .mu.T.
Miklich et al. Found that the 1/f flux noise at low frequencies (f) increased substantially when SQUIDs were cooled in static magnetic fields comparable to that of the Earth (A. H. Miklich et al., Appl. Phys. Lett. 64:3494, 1994). Because the spectral density of the flux noise scales linearly with the magnitude of the ambient magnetic field, B, the noise is attributed to the thermally activated hopping of vortices among pinning sites in the high-T.sub.C superconducting film (M. J. Ferrari et al., J. Low Temp. Phys. 94:15, 1994).
Excess noise has been a serious limitation for SQUID-based devices. Noise arises from two sources. First, there is ambient noise arising from a variety of sources, such as power lines or electrical appliances, in the environment. Second, there is noise that arises from magnetic flux vortices penetrating a superconducting film as it is cooled in a static ambient magnetic field. Low levels of noise have only been achieved by surrounding the SQUID magnetometer with magnetic shielding that reduced the ambient magnetic field to below 1 .mu.T. To achieve very low noise levels magnetic shielding must be used, for example, as provided by The Berlin Magnetically Shielded Room (BMSR). The room was designed to provide electromagnetic shielding at frequencies from dc to Ghz. The BMSR is a cube with inner dimensions of 2.25 m.times.2.25 m.times.2.25 m and outer dimensions of 4.6 m.times.4.6 m.times.4.6 m. The shielding is provided by six layers of high permeability alloy and an eddy current shield of copper plates. Another type of static magnetic field shielding structures is a multi-layered mu-metal shield. Using these types of shielding apparatus, the most sensitive magnetometers have achieved magnetic field noise levels at about 30 fT Hz.sup.-1/2 at 1 Hz (R. Cantor et al., IEEE Trans. Appl. Supercond., 5:2927, 1995; E. Dantsker et al., Appl. Phys. Lett., 67:725, 1995), and at about 10 fT Hz.sup.-1/2 at frequencies higher than the onset of 1/f problems, (D. Drung et al., Appl. Phys. Lett., 68:1421, 1996).
Development of high-T.sub.C SQUIDs which excluded magnetic flux vortices when cooled and operated in the Earth's magnetic field would be a particularly important advance in the art.